Conventional mixers are widely used in radio-telephones and other applications requiring the generation of the product of two analog waveforms or signals. It is known from trigonometry that the product is a signal at the sum and difference of the frequencies of the two input signals, i.e., the "sum and difference frequencies." The process for obtaining the product can be called mixing or multiplication.
Idealy, mixers are double balanced, single balanced or not balanced. In a double-balanced mixer, only the product of the two signals, i.e., the sum and difference frequencies, are produced. On the other hand, a single-balanced mixer produces an output containing additional components at the frequencies of one of the input signals and its harmonics. In a mixer that is not balanced, the additional components include frequencies of both input signals and their harmonics. The latter type of mixer can be implemented by applying the two signals to a non-linear device, such as, for example, a forward-biased diode, which non-linearly combines the two input signals.
Depending on the application, mixers are also known as frequency translators, modulators, synchronous detectors and phase detectors, As mentioned above, mixers are usually employed in radio-telephones, in both conventional transmitters and receivers. IN typical transmitters, e.g., mixers are used to modulate RF carriers with audio signals. In a typical receiver of a radio-telephone, for example, an incoming radio frequency ("RF") signal is mixed, i.e., combined in a mixer, with an adjustable signal from a local oscillator ("LO") to produce a signal at an intermediate frequency ("IF"). Downstream, the receiver has one or more amplifiers, filters and other components for processing and demodulating the IF signal, and thereby extracting the information it carries. Changing the LO frequency tunes the receiver to different radio frequencies.
While generally suitable for such uses, known mixers can produce products containing unwanted components at various frequencies, sometimes including a direct current ("DC") offset. The unwanted components result from interfering signals or other undesired waveforms contained within the input signals to the mixer, or from distortion introduced within the mixer itself. The unwanted components in the output of the mixer can ripple through downstream electronic devices, further distorting signals being processed, and even impairing the receiver's performance.
In addition, signal levels within the mixer can shift the DC operating point or bias of electronic devices within the mixer. A mixer is typically designed to operate at a particular DC operating point, which is selected to allow the mixer to accept a broad range of input amplitudes, i.e., a maximum or near maximum "dynamic range," with a minimum of distortion for a particular application. Any shift of the operating point can reduce its dynamic range from that optimal level and thus increase signal distortions.
Filters downstream from the mixer have been employed in the prior art to remove unwanted components of the mixer output signal outside a band of interest (i.e., "out-of-band components"). Such filters do not, however, correct distortions within the band of interest (i.e., "in-band"). Also, even though the filters can remove the out-of-band unwanted components, they do so only after the signal has already passed from the mixer, and thus the prior art filters do not prevent the untoward effects of the unwanted components within the mixer itself.